It would be good to have a recursive flag that would operate on all the .hs and .lhs files under a single directory.

Haddock should emit the documentation about instances. For example, it's important to document that the Data.Map instance of Foldable only folds over the values and not the keys.

There should be an annotation to include a function's entire definition in the documentation. This would be useful for functions like

(.)

and

mapM

where the definition is the clearest possible documentation, and for QuickCheck properties that specify the behavior of a library.

There should be an option to include a simplified implementation of a function that is equivalent to the one in the code. For instance, instead of showing a complex implementation of List.length that makes use of stream fusion we could show a simple one based on foldl'.